Joining of difficult-to-weld materials and sintering of powders using a low-temperature vaporization material

ABSTRACT

The present invention discloses a process for sintering particles using a sintering aid. The sintering aid can be brought into contact with a plurality of particles to be sintered such that a mixture of the particles and the sintering aid is provided. The mixture of particles and the sintering aid is heated and at least part of the sintering aid is vaporized. Sintering of the particles to form a sintered component followed by cooling of the sintered component can complete the process, or in the alternative, a subsequent heating step or steps can be included whereby additional vaporization of the sintering aid can occur.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of U.S. patent application Ser. No. 12/327,385 filed Dec. 3, 2008, which claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No. 60/991,966 entitled “Joining of Difficult-to-Weld Materials,” filed Dec. 3, 2007, the disclosure of which is hereby incorporated herein by reference.

STATEMENT REGARDING GOVERNMENT INTERESTS

This invention was made with government support under Cooperative Agreement No. DE-FC26-05NT42465 awarded by the U.S. Department of Energy. The government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to a process for joining materials and, in particular, to a process for joining difficult-to-weld materials. In addition, the present invention relates to a process for sintering powders using a sintering aid.

BACKGROUND OF THE INVENTION

The manufacture of electrical power plants, petrochemical refineries, and other industrial facilities requires joining of various components. Joining of such components can be performed using welding, adhesives, threaded joints, flanges that can be bolted together, and the like. In many instances, the welding of components provides a sound engineering and economical method for joining said components, and in fact, the ability of a material to be welded can have a great impact on the material's commercial viability.

With demands for increasing the efficiency of electrical power plants, gas turbine engines, and the like, the need for the use of materials that can withstand ever-increasing high temperatures continues. For example, dispersion-strengthened alloys are known to exhibit excellent high-temperature properties and have shown potential for use in many high-temperature applications. Likewise, nickel-based alloys strengthened by internal precipitants, such as gamma prime, are currently used in the hot sections of gas turbines. However, alloys such as these can present problems with respect to traditional fusion welding techniques since the melting of the base material results in the destruction of the microstructure which provides the excellent high-temperature properties.

Heretofore, joining techniques for such alloys have included diffusion bonding, friction welding, and other solid-state welding processes. Diffusion bonding is a process wherein two nominally flat interfaces are joined at an elevated temperature using an applied pressure upon the interfaces to be joined. The diffusion bonding process affords the joining of dissimilar materials and/or similar materials wherein the melting of the base material has detrimental effects. However, the presence of oxide layers at the joining surfaces can affect the quality of the joint, thereby making sound, reproducible joints difficult to obtain.

A modified form of diffusion bonding is known as transient liquid phase (TLP) diffusion bonding wherein liquid-state diffusion bonding relies on the formation of a liquid phase provided by a bonding film that is inserted between the interfaces to be joined during an isothermal bonding cycle. The liquid phase subsequently diffuses into the base material and eventually solidifies as a consequence of continued diffusion into the bulk material at the isothermal temperature. The liquid phase enhances dissolution and/or disruption of any oxide layer that may be present on the interfaces to be joined and, thereby, promotes intimate contact between said interfaces, possibly dissolving a portion of the metal present at the joint faces into the joint. As such, the presence of the bonding film and thus the liquid phase reduces pressure and time that may be required for diffusion bonding. However, methods to TLP diffusion bond dispersion-strengthened high-temperature alloys and gamma prime nickel-based alloys have met with limited success. Therefore, a process for bonding of such alloys and/or a bond foil having a composition that affords improved bond joints is desirable. In addition, a clamping device that affords the application of applied stress to the components to be joined is desirable.

The sintering of powders also includes many aspects that are present during the diffusion bonding of materials. For example, sintering involves heating of powders below their melting point until the particles adhere to each other. In addition, an oxide layer can be present on the surface of the particles and cause difficulties in proper adherence therebetween.

Various components can be advantageously made using a sintering process. For example, sintered bronze can be used as a bearing material, since porosity within the sintered component allows for lubricants to flow therethrough and remain captured therein. As such, a self-lubricating structure can be provided. In addition, materials having a high melting point such as tungsten can be sintered into a suitable shape when alternative manufacturing techniques are not available. In some instances, very low porosity can be obtained, and additional heating and/or pressurizing steps can be included in order to obtain a more dense structure. As with diffusion bonding, a process that affords for improved sintering of powders is desirable.

SUMMARY OF THE INVENTION

The present invention discloses a process for joining materials. The process can include providing a first component with a first joint face and a second component with a second joint face. The first joint face and the second joint face can be prepared for bonding, and a bonding layer can be provided. The first component, second component, and bonding layer can be assembled such that the first joint face is oppositely disposed from the second joint face with the bonding layer located at least partially therebetween. In addition, a force can be applied to the assembly of the first component, second component, and bonding layer such that the first joint face is compressed against the second joint face with the bonding layer therebetween. In some instances, the bonding layer can be a bonding foil, and the bonding foil may or may not be a zinc foil.

A thermal treatment can be applied to the first joint face and the second joint face with the bonding layer therebetween, thereby affording for at least part of the bonding layer material to melt, the first joint face coming into intimate contact with the second joint face and forming a bond interface, and the first component being bonded to the second component across the bond interface, with at least part of the bonding layer vaporizing during the process. In addition, an atmosphere surrounding the first joint face and the second joint face with the bonding layer therebetween can be controlled before, during, and/or after the thermal treatment. In some instances, the thermal treatment can be a multiple-step thermal treatment to the first joint face and the second joint face with the bonding layer therebetween.

In addition to disclosing a process for joining materials, a process for joining particles is also disclosed. In particular, a process for sintering particles is provided where a plurality of particles are provided along with a sintering aid. The sintering aid is brought into contact with the plurality of particles such that a mixture of the particles and the sintering aid is provided. The mixture of particles and the sintering aid is heated, and at least part of the sintering aid is vaporized. Sintering of the particles to form a sintered component followed by cooling of the sintered component can complete the process, or in the alternative, a subsequent heating step or steps can be included whereby additional vaporization of the sintering aid can occur.

The sintering aid can be a low melting point metal and/or a low melting point alloy. In addition, the sintering aid can be brought into contact with the plurality of particles by mixing the plurality of particles to be sintered with a plurality of sintering aid particles, coating the plurality of particles with the sintering aid, passing a vapor of the sintering aid past the plurality of particles, and/or passing a liquid of the sintering aid past the plurality of particles. It is appreciated that with the sintering aid in contact with the plurality of particles, heating of the mixture of particles and sintering aid before, during, and/or after the sintering process can at least partially melt the sintering aid and disrupt any oxide film that is present on the surface of the plurality of particles. Furthermore, melting of the sintering aid can enhance wetting between surfaces of adjacent particles, thereby increasing sintering kinetics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a process according to an embodiment of the present invention;

FIG. 2 is a side view of a clamping device that can be used with an embodiment of the present invention;

FIG. 3 is a schematic diagram illustrating sintering of a plurality of particles;

FIG. 4 is a schematic diagram illustrating the presence of an oxide film on surfaces of adjacent particles;

FIG. 5 is a schematic diagram illustrating the oxide film shown in FIG. 4 having been at least partially disrupted;

FIG. 6A is a schematic diagram illustrating particles of a sintering aid mixed with particles to be sintered;

FIG. 6B is a schematic diagram illustrating particles to be sintered having been coated with a sintering aid;

FIG. 6C is a schematic diagram illustrating vapor of a sintering aid passing through a plurality of particles;

FIG. 7 is a schematic diagram illustrating a process for sintering particles using a sintering aid;

FIG. 8A is a schematic diagram illustrating percent porosity as a function of time during a sintering process with and without the use of a sintering aid; and

FIG. 8B is a schematic diagram illustrating sintering kinetics during a sintering process with and without the use of a sintering aid.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention discloses a process for joining materials using diffusion bonding, TLP diffusion bonding, and modifications thereof. As such, the process has utility as a process for joining materials and, in particular, for joining difficult-to-weld materials.

The process includes providing components to be joined, for example, a first component having a first joint face and a second component having a second joint face. The first joint face and/or the second joint face can be prepared for bonding to each other. In some instances, the first joint face and/or the second joint face are machined. Optionally, the first joint face and/or the second joint face can be polished or otherwise finished in addition to, or in place of, the machining.

A bonding layer can be provided. In some instances, the bonding layer is a bonding foil. The bonding foil can be a metallic foil such as a zinc foil; the term “zinc foil” for the purposes of the present invention includes foil made from high-purity zinc, commercial pure zinc, zinc alloys, and the like. For example, the zinc foil can be made from zinc alloyed with aluminum, copper, lead, magnesium, nickel, iron, and/or tin. It is appreciated that the bonding layer can be a paste that is applied to the first joint face and/or second joint face or a coating that has been applied to one of the joint faces. The coating can be applied by any method known to those skilled in the art, illustratively including sputtering, chemical vapor deposition, physical vapor deposition, and the like.

The first component, second component, and bonding layer can be assembled such that the first joint face is oppositely disposed from the second joint face and at least part of the bonding layer is located therebetween. A force can be applied to the assembly of the first component and the second component with the bonding layer therebetween and afford for the first joint face to be compressed against the second joint face and the bonding layer.

The first joint face and the second joint face with the bonding layer located therebetween can be subjected to a thermal treatment, the thermal treatment affording for at least part of the bonding layer material to melt and the first joint face bonding to the second joint face, with at least part of the bonding layer vaporizing, possibly after diffusing through the structure being joined. In some instances, the thermal treatment can be a multiple-step thermal treatment, or in the alternative, a single-step thermal treatment where the temperature of the joint region is continuously increased to a final temperature. In the instance of a multiple-step thermal treatment, the thermal treatment can include a first step that includes heating the first joint face and the second joint face with the bonding layer therebetween to a first temperature, followed by holding at the first temperature for a predetermined amount of time, and a second step that includes heating to a second higher temperature followed by holding at the second temperature for a predetermined amount of time.

The first temperature may or may not be higher than the melting point or solidus temperature of the bonding layer, and the second temperature may or may not be higher than a recrystallization temperature of the first component and/or the second component. In this manner, the first temperature may result in the melting of the bonding layer, and the second temperature may result in grain growth across a bond interface between the first and second components. It is appreciated that melting of the bonding layer can afford for wetting of the first and/or second joint face and/or disrupting of any surface oxide on the first joint face, second joint face, and/or bonding layer, possibly dissolving a portion of the metal present at the joint faces into the joint.

It is appreciated that grain growth across the bond interface can result in improved bond joint quality and strength. It is further appreciated that cold-working of the first and/or second component proximate the first joint face and/or second joint face, respectively, can enhance grain growth across the bond interface. In the alternative, the bonding of components having different compositions can afford for one or more concentration gradients across the bond interface, the concentration gradient(s) enhancing grain growth across the bond interface and cold-working of the first and/or second component not being required.

In some instances, an atmosphere surrounding the first joint face and the second joint face with the bonding layer therebetween can be controlled. The atmosphere can be controlled by purging with an inert gas and/or by pulling or drawing a vacuum on a chamber in which the first joint face and the second joint face with the bonding layer therebetween is contained within. The inert gas can include a reducing gas such as hydrogen, for example argon with 5 volume percent hydrogen. It is appreciated that terms such as “draw,” “drawing,” “pull,” “pulled,” and “pulling” are terms of art when used in the context of a vacuum and refer to the removal of atoms and/or molecules from an enclosed container, i.e., a chamber, and the establishment of a pressure that is less than atmospheric pressure therewithin.

Control of the atmosphere surrounding the first joint face and the second joint face with the bonding layer therebetween can be combined with the multiple-step thermal treatment. For example and for illustrative purposes only, a chamber containing the first joint face and the second joint face with the bonding layer therebetween can be purged with an inert gas before and/or during the first step, followed by establishing a vacuum before and/or during the second step.

During at least part of the thermal treatment, with or without the atmosphere control, contact between the interfaces to be bonded is sufficient such that diffusion takes place therebetween, and a sound metallurgical bond is provided. As stated above, the thermal treatment can include a step that affords grain growth across the bond interface, the bond interface being defined herein as an interface between two components to be joined, across which diffusion occurs to form a bonded joint. In this manner, a process wherein joints having acceptable room-temperature and/or high-temperature properties is provided.

Components that can be joined using the process disclosed herein range from typical metals and alloys used for fabricating structures to difficult-to-weld metals and alloys. For example and for illustrative purposes only, materials such as the commercial alloys MA956, PM2000, CM247LC, APMT, and the like can be joined to themselves and/or to other materials. It is appreciated that the MA956 alloy is an oxide dispersion-strengthened (ODS) alloy having a nominal chemical composition of Fe-20Cr-4.5Al-0.5Ti-0.5Y₂O₃ (wt %); the PM2000 alloy is also an ODS alloy having a nominal chemical composition of Fe-20Cr-5.5Al-0.5Ti-0.5Y₂O₃ (wt %); the CM247LC alloy is a gamma prime-strengthened alloy having a nominal composition of Ni-8.1Cr-9.2Co-0.5Mo-9.5W-3.2Ta-0.7Ti-5.6Al-0.01Zr-0.01B-0.07C-1.4Hf (wt %); and the APMT alloy is a dispersion-strengthened powder metallurgy alloy having a nominal composition of Fe-21Cr-3Mo-5Al (wt %). It is further appreciated that these alloys, and other alloys joined by the process disclosed herein, can have other incidental impurities and additional alloying elements.

The process can also include the use of a fixture device for holding the components to be joined in an appropriate orientation with a desired stress applied thereon.

Similar to the use of a bonding layer to enhance the joining of two components, a sintering aid can be used to enhance the joining of adjacent particles during a sintering process. A process for sintering particles can include providing a plurality of particles and providing a sintering aid. The sintering aid can be brought into contact with the plurality of particles and thereby result in a mixture of the particles and the sintering aid. Heat can be applied to the mixture of particles and sintering aid, thereby affording for vaporization of at least part of the sintering aid.

The sintering aid can be selected from a low melting point metal, a low melting point alloy, and the like. In some instances, the sintering aid contains zinc and/or a zinc alloy. The mixture of the plurality of particles to be sintered and the sintering aid can be in the form of the particles to be sintered mechanically mixed with a plurality of sintering aid particles. In addition, the plurality of particles can be at least partially coated with the sintering aid, and/or a vapor and/or a liquid of the sintering aid can be afforded to flow past or through the plurality of particles.

In some instances, the mixture of particles to be sintered and the sintering aid can be heated after a sintered component has been made and cooled, thereby affording for additional vaporization of the sintering aid from the sintered component. The heating of the sintered component can occur in a controlled environment, e.g., a vacuum, an inert gas atmosphere, etc., which may or may not enhance the vaporization of the sintering aid.

In FIG. 1, a process for joining difficult-to-weld materials is illustrated generally at reference number 5. The process 5 includes providing components to be joined at step 10, wherein the components can be made from any alloy or combination of alloys, metals, etc., illustratively including oxide or other ceramic dispersion-strengthened alloys, directionally solidified alloys, internal precipitate-strengthened alloys, solid solution-strengthened alloys, castings, and the like.

Included in the process 5 is a bonding layer at step 20. The bonding layer can be a zinc foil or, in the alternative, made from a material not containing zinc so long as the material has a tendency to vaporize during thermal treatment of a joint region as taught below. For example, zinc has a vapor pressure of 0.13 kilopascal (kPa) (1 torr) at 487° C., 101.3 kPa (760 torr) at 907° C. and 10,132 kPa (7600 torr) at 1180° C. In addition, zinc has a melting point of 420° C., which is less than, or about the same as, other iron-zinc or nickel-zinc alloy or intermetallic melting temperatures. As such, a zinc foil will melt before or at the same temperature as other possible zinc-containing compounds in an iron-based or nickel-based component, and vaporization of at least part of the zinc foil will reduce or eliminate it from the joined structure. Vaporization may occur from near the joint itself, or from the surface of the joined structure after the zinc has diffused through the structure.

It is appreciated that other low boiling point elements can be used for the bonding layer. For example and for illustrative purposes only, foils can be made primarily from elements such as arsenic (T_(b)=610° C.), cadmium (T_(b)=765° C.), cesium (T_(b)=690° C.), magnesium (T_(b)=1110° C.), mercury (T_(b)=357° C.), phosphorus (T_(b)=283° C.), polonium (T_(b)=960° C.), potassium (T_(b)=770° C.), rubidium (T_(b)=700° C.), selenium (T_(b)=685° C.), sodium (T_(b)=890° C.), sulfur (T_(b)=445° C.), and/or tellurium (T_(b)=962° C.) where T_(b) is the boiling point of the given element at atmospheric pressure. It is appreciated that some of these materials are considered poisonous, fire hazardous, and/or radioactive and thus may limit their use as a bonding layer but have in common with zinc a relatively low vaporization temperature.

After the components and the bonding layer have been provided, the components and said bonding layer are assembled at step 30. It is appreciated that the components to be joined can have joint faces that have been properly prepared, for example, by machining and/or polishing or other surface preparation, and the bonding layer can be dimensioned to fit between the joint faces. The bonding layer can have a thickness between 1 μm and 1 mm, inclusive. In some instances, the bonding layer has a thickness between 5 μm and 200 μm and in other instances can be between 20 μm and 50 μm. Assembly of the components with the bonding layer at step 30 includes bringing the joint faces to be bonded into intimate contact with the bonding layer, the joint faces being oppositely disposed from each other with the bonding layer therebetween. In addition, pressure or an applied stress can be applied to the components such that the interfaces to be bonded and the bonding layer are under compression.

The pressure can assist in the breaking up or disruption of any oxide scale that is present on the first joint face, second joint face, and/or bonding layer and possibly dissolving a portion of the metal present at the joint faces into the joint. It is appreciated that the pressure can be applied with a fully articulated press or assembly device that affords for the first joint face and the second joint face to be easily aligned with each other and thus provide for intimate contact therebetween once the bonding layer has melted and diffused into the components and/or been vaporized away from the joint region.

An assembly device shown generally at reference numeral 80 in FIG. 2 can be included to assist in the assembly of the components to be joined. As shown in FIG. 2, the assembly device 80 can include a body 100 having a top portion 110 and a bottom portion 120. Within the body 100 can also be at least one cavity 130 that affords for the placement of a first component 210 to be joined to a second component 220. A bonding foil 230, for example, a zinc foil, can be placed between the first component 210 and the second component 220 as illustrated in the figure. The first component 210 can have a first joint face 212 and the second component 220 can have a second joint face 222. As stated above, the first joint face 212 and/or the second joint face 222 can be prepared for bonding to the oppositely disposed joint face.

Proximate to the top portion 110 is a pressure application member 112. In some instances, the pressure application member 112 can be a threaded bolt, screw, and the like. The pressure application member 112 can have a pressure end 114 that can be moved in a back and forth direction 1. In addition to the body 100 and the pressure application member 112, a hemispherically shaped cap 140 can be placed between the pressure end 114 of the pressure application member 112 and the first component 210 to be joined. Likewise, a second hemispherical cap 150 can be placed between the member 100 and the second component 220 to be joined. It is appreciated from FIG. 2 that the cap 140 and the cap 150 are placed at distal ends or locations from first joint face 212 and second joint face 222, respectively. It is also appreciated that the pressure end 114 of the pressure application device 112 has a shape that is complementary with the hemispherical cap 140 as illustrated in FIG. 2. The body 100 can also have a machined region 122 that is complementary to the spherical portion of the hemispherical cap 150.

The hemispherical caps 140 and 150 can be made from any material known to those skilled in the art, illustratively including high-temperature alloys, alumina, silica, and the like. In some instances, the hemispherical cap 140 and 150 has a hemisphere diameter that is generally equivalent to the diameter of a rod, tube, and the like that is to be joined; however, this is not required. If the components to be joined have a cross-sectional polygon shape such as a square, rectangle, and the like, the cap 140 and cap 150 can be manufactured such that one end is complementary to the pressure end 114 and/or machined region 122 and the other end is complementary to the components to be joined.

The arcuate surface of the hemispherical cap 140 and/or 150 affords for the components to fully articulate, or move, independently from the body 100 and the pressure applied by the pressure application device 112. This articulation or movement can be critical since the joint diffusion zone that results in the bond can be relatively thin and the interfaces to be joined are preferably in intimate contact along the complete surfaces of the joint. Without the articulation, the joints can become cocked, misaligned, etc., with force on one portion being greater than another portion and the interfaces to be joined not being parallel with each other.

It is appreciated that the member 100 and the pressure application device 112 can be made from any material known to those skilled in the art for use at generally high temperatures, such as molybdenum, niobium, other metals having high-temperature strength, high-temperature nickel-based alloys, high-temperature iron-based alloys, high-temperature cobalt-based alloys, ceramics, metal matrix composites, and the like.

Returning to FIG. 1, after the components to be joined have been assembled at step 30, the atmosphere surrounding the first joint face 212 and the second joint face 222 with the bonding foil 230 therebetween, i.e., the joint region, can be controlled at step 40. In some instances, the assembly of the components to be joined is placed within an enclosed chamber such that the chamber can be evacuated and/or purged with an inert gas. In other instances, the region wherein the joint is to occur is enclosed without the entire assembly being placed in a chamber. The control of the atmosphere surrounding the joint region can be critical and, in some instances, is provided by a high vacuum. The atmosphere can also be controlled by purging the joint region with an inert and/or reducing gas, or inert gas mixture, illustratively including argon, nitrogen, mixtures of those gases with hydrogen, and the like. It is appreciated that the atmosphere can be controlled by a combination of vacuum and gas purging. A vacuum of less than 10⁻⁴ kPa (10⁻³ millibar) can assist in the vaporization of the bonding foil material away from the joint region and/or aid in decomposing of any oxide scale that is present.

An oxygen getter can be placed proximate to the joint region and/or within an enclosed chamber that contains the joint region such that excess oxygen within the atmosphere is reduced. Any oxygen getter known to those skilled in the art can be used, illustratively including an oxygen getter made from zirconium, aluminum, tantalum, titanium, and the like. In some instances, the oxygen getter is in the form of a sponge or some other high-surface-area structure. In addition, the joint region can be wrapped with oxygen getter foils such as aluminum, zirconium, tantalum, titanium, and the like.

A thermal treatment of the joint region can be provided at step 50. The thermal treatment can result in the heating of the joint region, and the heating can be provided by thermal resistance, thermal resistance furnaces, induction heating, radiant heating, and the like. The thermal treatment can include a series of time-temperature steps, such as a ramp up to a first temperature, holding the first temperature for a predetermined amount of time, ramp up or down to a second temperature, holding at the second temperature for a predetermined amount of time, ramp up or down to a third temperature, holding at the third temperature at a predetermined amount of time, and so on.

For example and for illustrative purposes only, a first component 210 made from the CM247LC alloy can be joined to a second component 220 made from the APMT alloy using a zinc foil. A chamber surrounding the first joint face 212 and the second joint face 222 with the zinc foil 230 therebetween can be purged with an argon+5% hydrogen gas and held at a pressure of between 10 to 304 kPa (0.1 to 3 atmospheres) while the joint is heated to 700° C. and held for 1 hour. This initial step can result in the melting of the zinc foil and disruption or dissolving of oxide surfaces at the first joint face 212 and/or second joint face 222, possibly dissolving a portion of the metal present at the joint faces into the joint. Thereafter, a high vacuum, for example a vacuum of 10⁻⁷ kPa (10⁻⁶ millibar), can be pulled around the joint region and the temperature increased to 1214° C. and held for 24 hours. During this second thermal step, grain growth and interdiffusion across the joint interface can be promoted and at least part of the zinc from the zinc foil vaporized, possibly after diffusing through the structures being joined to the surface of the structures.

It is appreciated that the second thermal processing step can include holding the joint region at the second temperature for a shorter amount of time, for example, 1 hour and thereafter reducing the vacuum and providing an Ar+5% H₂ gas. Such an alternative thermal treatment can reduce vaporization losses from assembly devices, furnace tubes, clamps, joint rods, and the like. It is appreciated that with any of these thermal treatment steps, the atmosphere can be further controlled by the introduction of oxygen getter materials therein.

Additional thermal treatment steps can be included, such as additional heating steps and subsequent cooling steps. Heat treatment, stress relief, and/or aging thermal treatment steps can be included along with the joining steps and still fall within the scope of the invention.

In this manner, a first component can be joined to a second component using a bonding layer that melts at a temperature that is lower than the melting temperature of the first component and the second component. In addition, at least part of the melted bond layer can vaporize after it wets and dissolves at least a portion of a first joint face and/or a second joint face, possibly after diffusing through the structures being joined.

In FIG. 3, a schematic diagram illustrating sintering of a plurality of particles is shown. A plurality of particles to be sintered 300 can be accumulated within a preform (not shown) with a plurality of voids 310 being present between the particles 300. Upon the application of heat and/or pressure, the porosity 310 between the particles 300 can be reduced and regions of contact between the particles resulting in diffusion therebetween and their adherence to each other. In some instances, generally all of the porosity 310 can be removed with subsequent grain growth and/or recrystallization occurring and affording for enlarged grains 320.

In some instances, an oxide film 302 can be present on at least part of a surface of the particles 300 as illustrated in FIG. 4. It is appreciated that the oxide film 302 can act as a diffusion barrier between adjacent particles 300 and thereby retard the sintering process. In contrast, the use of a sintering aid as disclosed herein can result in the oxide film 302 being broken up into discrete oxide regions 302 as illustrated in FIG. 5. It is appreciated that the breaking up of the oxide 302 is a result of the sintering aid being present within the mixture of the plurality of particles 300 to be sintered.

For example and for illustrative purposes only, FIG. 6A illustrates a plurality of sintering aid particles 330 having been admixed with the plurality of particles 300 before the sintering process is initiated. In the alternative, a coating of the sintering aid 330 can be present on at least a portion of the surface of the particles 300 and/or the oxide film 302 as illustrated in FIG. 6C. In another alternative, a vapor and/or liquid of the sintering aid 330 can be allowed to flow past or through a plurality of particles 300 and/or the porosity 310 as illustrated in FIG. 6C. In this manner, the sintering aid 330 is brought into contact with the plurality of particles 300 and heating of the sintering aid 330 can result in at least partially melting thereof and afford for disruption of the oxide 302 and/or wetting of adjacent surfaces of particles 300. In addition, at least part of the oxide 302 and/or particle 300 can dissolve into the melted sintering aid 330.

Referring to FIG. 7, a method of sintering powders with the use of the sintering aid is shown generally at reference numeral 40. The process 40 can include providing powders to be sintered at step 400 and a sintering aid at step 410. The powders to be sintered and the sintering aid are mixed at step 420. For the purposes of the present invention, the mixing of the powders and the sintering aid can include a mechanical mixture of the powders and particles of the sintering aid, coating of the powders with the sintering aid, flowing of a vapor and/or a liquid of the sintering aid past or through the powders, and the like. At step 430 the powders are sintered by heating and/or applying pressure thereto, with vaporization of at least part of the sintering aid optionally occurring at step 432. It is appreciated that the sintering aid can vaporize while it is initially on the surface of a particle and/or after having diffused into the particle and reached a surface thereof. In some instances, the powders with the sintering aid can be contained within a preform that can be used to produce a desired component.

After sintering of the powders at step 430, cooling of a sintered component made from the sintered powders occurs at step 440. In some instances, an optional heating of the sintered component can occur at step 450 and thereby allow for additional vaporization of the sintering aid to occur. It is appreciated that one or more heating steps can occur with or without a controlled environment in order to maximize the properties of the sintered component and/or remove additional sintering aid by vaporization. In addition, it is further appreciated that the vaporization of the sintering aid can occur after the component is sintered. For example and for illustrative purposes only, the powders can be sintered while under external pressure in order to increase the kinetics of the sintering process, but which reduces the vaporization of the sintering aid. Thereafter, the sintered component can be heated without the presence of the external pressure, or possibly in vacuum, and thereby afford for vaporization of at least part of the sintering aid. The heating of the sintered component can also include an additional heating step after the sintered component has been cooled, or in the alternative, a continuous heating of the sintered component with the removal of the external pressure. For the purposes of the present invention, the term “external pressure” is defined as pressure greater than atmospheric pressure.

With reference to FIGS. 8A and 8B, the result of using the sintering aid is shown. In particular, FIG. 8A illustrates the use of the sintering aid affording increased kinetics with respect to the reduction of percent porosity as a function of time during the sintering process. Likewise, FIG. 8B illustrates an increase in the average grain size and/or percent recrystallization as a function of sintering time. As stated above, the sintering aid enhances the sintering process by disruption of an oxide film that is present on the surface of the particles to be sintered and/or enhancing wettability between adjacent particles. In this manner, diffusion between adjacent particles can occur at an earlier stage of the sintering process and/or be enhanced by the presence of the sintering aid.

Similar to the materials used for the bonding layer described above, it is appreciated that other low boiling point elements can be used as the sintering aid. For example and for illustrative purposes only, the sintering aid can be made primarily from elements such as arsenic, cadmium, cesium, magnesium, mercury, phosphorus, polonium, potassium, rubidium, selenium, sodium, sulfur, tellurium, and/or zinc. As stated above, it is appreciated that some of these materials are considered poisonous, fire hazardous, and/or radioactive and thus may limit their use as a sintering aid. Zinc is a preferred sintering aid as in the case of the material used for the bonding layer described above because of a relatively low vaporization temperature, its ease of handling, and commercial availability.

The foregoing drawings, discussion, and description are illustrative of specific embodiments of the present invention, but they are not meant to be limitations upon the practice thereof. Numerous modifications and variations of the invention will be readily apparent to those of skill in the art in view of the teaching presented herein. It is the following claims, including all equivalents, which define the scope of the invention. 

1. A process for sintering particles, the process comprising: providing a plurality of particles; providing a sintering aid; bringing the sintering aid into contact with the plurality of particles and making a mixture of particles and sintering aid; heating the mixture of particles and sintering aid whereby vaporizing at least part of the sintering aid and sintering of the particles to form a sintered component occurs; and cooling of the sintered component.
 2. The process of claim 1, wherein the sintering aid is selected from the group consisting of a low melting point metal and a low melting point alloy.
 3. The process of claim 2, wherein the sintering aid contains zinc.
 4. The process of claim 3, wherein the sintering aid is a zinc alloy.
 5. The process of claim 1, wherein the plurality of particles is metallic.
 6. The process of claim 5, wherein the plurality of particles is metallic alloy particles.
 7. The process of claim 1, wherein bringing the sintering aid into contact with the plurality of particles is coating the plurality of particles with the sintering aid.
 8. The process of claim 1, wherein bringing the sintering aid into contact with the plurality of particles is mechanically mixing the plurality of particles with a plurality of sintering aid particles.
 9. The process of claim 1, wherein bringing the sintering aid into contact with the plurality of particles is passing a vapor of the sintering aid past the plurality of particles whereby the vapor comes into contact with a surface of at least part of the plurality of particles.
 10. The process of claim 1, wherein bringing the sintering aid into contact with the plurality of particles is passing a liquid of the sintering aid past the plurality of particles whereby the liquid comes into contact with a surface of at least part of the plurality of particles.
 11. The process of claim 1, wherein the mixture of particles and sintering aid are heated while under an external pressure.
 12. The process of claim 11, wherein the heating continues and the external pressure is removed after the forming of sintered component occurs.
 13. The process of claim 12, wherein at least part of the sintering aid is vaporized after the external pressure is removed.
 14. A process for sintering particles, the process comprising: providing a plurality of metallic particles; providing a sintering aid in the form of a low melting material; bringing the sintering aid into contact with the plurality of particles and making a mixture of particles and sintering aid; heating the mixture of particles and sintering aid; vaporizing at least part of the sintering aid; sintering the particles to form a sintered component; and cooling of the sintered component.
 15. The process of claim 14, wherein the sintering aid contains zinc.
 16. The process of claim 14, wherein the sintering aid is in the form of particles containing zinc.
 17. The process of claim 14, wherein bringing the sintering aid into contact with the plurality of particles is mechanically mixing the plurality of particles with the sintering aid particles containing zinc.
 18. The process of claim 14, wherein bringing the sintering aid into contact with the plurality of particles is coating the plurality of particles with the sintering aid.
 19. The process of claim 14, wherein bringing the sintering aid into contact with the plurality of particles is passing a vapor of the sintering aid past the plurality of particles whereby the vapor comes into contact with a surface of at least part of the plurality of particles.
 20. The process of claim 19, further including heating the sintered component in a vacuum after it has cooled. 